The present invention relates to a mobile communications system and, more particularly, to a mobile communications system having a control channel for transmitting and receiving control signals.
A mobile communications system such as a cordless telephone system is made up of fixed units connected to a public telephone network, and portable units connectable to the fixed units over radio channels and each including a handset. The radio channels include speech channels adapted for conversations and at least one control channel adapted for the transmission and reception of control signals. The frequencies necessary for the speech and control channels are generated by a synthesizer which is installed in each of the fixed and portable units. That is, a cordless telephone system adopts a multichannel access system which is implemented with synthesizers.
The control channel is used to control the connection of subscriber lines and the portable units, e.g., origination and reception of a call and designation of a speech channel. Those fixed and portable units which are in a stand-by state are turned to the frequency of the control channel. In any of the fixed and portable units held in such a condition, if power sources for a transmitter section and a power amplifier section following the transmitter section are turned ON, an electromagnetic wave will be unwantedly radiated to the control channel, thereby preventing another fixed or portable unit from using the control channel. While such a problem may be solved by shutting off the power supply to the transmitter section and power amplifier section during the stand-by state, this kind of scheme brings about another problem that the synthesizer built in the transmitter section needs a substantial period of time to start its operation and become stable in the event when a conversation is required. In the light of this, it has been customary to, under the stand-by condition, tune the frequency of the synthesizer to that of the control channel and shut off the power supply to the power amplifier section while maintaining the power supply to the transmitter section.
However, such one-sided shut off of the power supply is incapable of fully eliminating the leakage of electromagnetic wave, even if an extra circuit for minimizing the leakage of electromagnetic wave is installed in the power amplifier section as in a prior art system. Also, use of such an extra circuit complicates circuit construction. Especially, the leakage of electromagnetic wave is fatal to a cordless telephone system which has a relatively narrow communication zone (e.g. about several tens of meters), because the fixed and portable units and/or the portable units themselves are frequently used in proximity to each other. In such a case, the leakage of electromagnetic wave to the control channel would adversely affect the operation of the entire system.